The long-term objective of this proposal is to increase our understanding of the relationship of Ca2+ waves, altered intracellular Ca2+ cycling in the mechanisms of cardiac arrhythmias. The applicant proposes that the identification of the cellular and subcellular mechanisms of Ca2+ waves is crucial for the understanding of the initiating events of both normal and abnormal rhythms. Intracellular Ca2+ homeostasis will be studied on three levels including: 1) single Purkinje cells isolated from the subendocardium of normal and arrhythmogenic infarcted canine hearts; 2) aggregates or "strands" of Purkinje cells isolated from normal and arrhythmogenic infarcted canine hearts; and 3) working trabeculae isolated from the right ventricles of normal and arrhythmogenic infarcted rat hearts. Specific aim 1 will test the hypotheses that Purkinje cells that survive after a myocardial infarction have abnormal Ca2+ cycling and that this altered calcium cycling contributes to abnormal (i.e., arrhythmogenic) electrical activity. This will involve the measurement of whole-cell Ca2+ transients (fura-2 fluorescence) and whole-cell Ca2+ currents via standard whole-cell voltage clamp techniques. Specific aim 2 tests the hypothesis that Ca2+ oscillations are important regulatory signals both within a single cell as well as between cells. Aggregates of Purkinje cells will be studied with the perforated patch technique and intracellular Ca2+ will be visualized as fluo-3 fluorescence. Specific aim 3 tests the hypotheses that the initiation and propagation of triggered propagated contraction (TPCs) are due to abnormal electrical activity and propagating Ca2+ waves, and that the sarcolemma modulates calcium propagation by its ability to buffer Ca2+ and depolarization induced Ca2+ release. A multicellular preparation (i.e., working trabeculae) will be used to study Ca2+ waves that are visualized as fluo-3 fluorescence.